The invention relates to a process and a catalyst for preparing C2-oxygenates from synthesis gas, i.e. CO/H2 gas mixtures.
The heterogeneously catalyzed conversion of synthesis gas into hydrocarbons (Fischer-Tropsch synthesis) or into methanol has been known since the 1920s. It was discovered only in 1975 that rhodium catalysts can convert synthesis gas L directly into C2-oxygenates.
In principle, the direct reaction of synthesis gas over rhodium catalysts can lead to a broad range of products. Thus, methane and higher hydrocarbons and also oxygenates such as methanol, ethanol, higher alcohols, diols, acetaldehyde and acetic acid are formed over unpromoted Rh/SiO2 catalysts. Both the selectivity and the activity of rhodium catalysts can be considerably modified by doping with other elements or by appropriate selection of the support. For example, doping with lithium or potassium serves to suppress methane formation, doping with manganese gives a considerable increase in activity without the selectivity being significantly influenced, and the use of CeO2 instead of SiO2 as support results in a significant increase in the selectivity of ethanol formation. Furthermore, the product distribution depends on reaction parameters such as pressure, temperature, conversion and CO/H2 ratio.
EP-A-0 010 295 describes a process for preparing ethanol from synthesis gas, in which the reaction is carried out over a supported rhodium catalyst comprising, as cocatalyst, one or more of the elements zirconium, hafnium, lanthanum, platinum, chromium and mercury. EP-A-0 079 132 relates to a process for preparing oxygenated hydrocarbons by catalytic reaction of synthesis gas over a supported catalyst comprising, as active components, rhodium, silver, zirconium and molybdenum and also, if desired, iron, manganese, rhenium, tungsten, ruthenium, chromium, thorium and potassium. The preferred support material is silicon dioxide. J5 9078-130-A relates to a process for preparing acetic acid, acetaldehyde and ethanol by reaction of synthesis gas over a rhodium catalyst comprising manganese, zirconium and at least one alkali metal as promoters.
The direct, heterogeneously catalyzed gas-phase synthesis of acetic acid from synthesis gas over rhodium catalysts has been intensively researched, especially in the years 1980 to 1986 as part of the Japanese C1 project xe2x80x9cResearch and Development Program for New Technologies to Produce Basic Industrial Chemicals from Carbon Monoxide and other Chemicalsxe2x80x9d. During the course of this work on the synthesis of acetic acid, the influence of about 60 elements as promoters on the activity and selectivity of heterogeneous rhodium catalysts was systematically examined. It was found that promoters such as Mg, Sc, Ti, V, Cr, Mn, Mo or La increase the catalytic activity, while promoters such as Li, K, Zr or Ir lead to an increase in the selectivity of acetic acid formation. At the end of the project, an optimized Rhxe2x80x94Mnxe2x80x94Irxe2x80x94Lixe2x80x94K/SiO2 catalyst enabled an acetic acid selectivity of 71% to be achieved at a space-time yield of 344 g/l*h. A further catalyst which was not described in more detail but gave an overall selectivity to acetic acid +acetaldehyde of 90% at a space-time yield of 480 g/l*h was also developed.
The results of the Japanese Cl project have been compiled in the book xe2x80x9cProgress in Cl Chemistry in Japanxe2x80x9d, Elsevier, 1989, with specifically the work on the preparation of acetic acid from synthesis gas being described in Chapter 6 (pages 287 to 330). In the screening tests on catalyst optimization, not only was the influence of various promoters on the catalyst performance elucidated, but a number of further parameters such as the test conditions (P, T, SV, CO/H2 ratio), partial poisoning with sulfur, the support composition and pore structure, the Rh particle size and the conditions in the drying and the reduction of the catalyst were also examined. In addition, the reaction kinetics over an Rhxe2x80x94Mnxe2x80x94Irxe2x80x94Lixe2x80x94K/SiQ2 catalyst and the ageing behavior of an Rhxe2x80x94Mnxe2x80x94Irxe2x80x94Li/SiO2 catalyst were examined.
It is an object of the present invention to provide a process and a catalyst for preparing C2-oxygenates, which give the desired products with high conversions and high selectivities at a high catalyst activity and operating life.
We have found that this object is achieved by a process for preparing C2-oxygenates by reaction of CO and H2 over a rhodium-containing supported catalyst, in which the catalyst comprises, based on the total weight,
from 0.01 to 10% by weight of rhodium,
from 0.001 to 10% by weight of zirconium,
from 0.01 to 5% by weight of iridium,
from 0.01 to 10% by weight of at least one metal selected from among copper, cobalt, nickel, manganese, iron, ruthenium and molybdenum,
from 0.01 to 10% by weight of at least one alkali metal or alkaline earth metal selected from among lithium, sodium, potassium, rubidium, magnesium and calcium, on an inert support.
It has been found, according to the present invention, that the above catalyst displays a very good performance spectrum in the conversion of synthesis gas into C2-oxygenates. The catalyst displays a high activity and selectivity, even when the rhodium content is very low.
The catalyst support is preferably selected from among SiO2, Al2O3, TiO2, zeolites, activated carbon, diatomaceous earth and mixtures thereof.
The alkali or alkaline earth metal or metals is/are preferably selected from among lithium, potassium and magnesium.
The metal or metals is/are preferably selected from among copper, manganese, iron, ruthenium and molybdenum.
A preferred catalyst comprises
from 0.1 to 5% by weight, in particular from 0.5 to 3.5% by weight, of rhodium,
from 0.05 to 5% by weight, in particular form 0.08 to 4.5% by weight, of zirconium,
from 0.1 to 3.5% by weight, in particular from 0.2 to 2.5% by weight, of iridium,
from 0.1 to 5% by weight, in particular from 0.2 to 1.5% by weight, of at least one metal selected from among copper, cobalt, nickel, manganese, iron, ruthenium and molybdenum,
from 0.05 to 5% by weight, in particular from 0.1 to 1.2% by weight, of at least one alkali metal or alkaline earth metal selected from among lithium, sodium, potassium, rubidium, magnesium and calcium, on an inert support.
The catalyst of the present invention can be obtained by impregnating the inert support with catalyst metal compounds, in particular catalyst metal salts, dissolved in aqueous or organic solvents, drying and calcining the impregnated support and subsequently reducing it. The impregnation can be carried out in any desired manner, for example by steeping or spraying. Preference is given to using catalyst metal salts which are soluble in water or organic solvents such as ethanol. The order in which the support is impregnated with the catalyst metal compounds can be chosen freely. Impregnation can be carried out successively or simultaneously with all components.
According to the present invention, the catalyst is used in the reaction of synthesis gas, in particular to give C2-oxygenates such as ethanol, acetaldehyde and acetic acid.
The reaction is preferably carried out at a pressure in the range from 1 to 100 bar and at a temperature in the range from 200 to 400xc2x0 C.
The molar ratio of H2 to CO is preferably 10-0.05:1.